A wide variety of fire extinguishing powders are known and have been studied for different purposes, for example for preventing ignition in an explosive gas mixture; for extinguishing a propagating flame; or for extinguishing burning material such as sodium metal. For example Bulewicz et al. (Arch. Combust. 1987 7(1-2)) describe the effects of powders of SiO.sub.2, CaCO.sub.3, NH.sub.4 H.sub.2 PO.sub.4, NaCl, and NaHCO.sub.3 of particle sizes 20-40 .mu.m on flame propagation and on explosion suppression. EP 0 309 881 (Shin-Etsu Handotai) describes use of a silica-based powder for extinguishing sodium and magnesium metal fires, the particles being porous and of size between 5 .mu.m and 5 mm; for use in fire extinguishers the preferred range is 5 .mu.m to 200 .mu.m, while particles smaller than 5 .mu.m are said to be not suitable. In contrast EP 0 638 335 (Fenwal) teaches the use of a dry powder, based on silica, alumina or titania, and of particle size below 20 .mu.m, preferably below 5 .mu.m. The use of pyrotechnically-initiated aerosol-generating formulations has been described by Kilbert et al. (Fire Technology, 1994, p.387), the aerosols having typical particle sizes between 1 .mu.m and 3 .mu.m, and Kilbert et al. also present data that indicates that the extinguishing effectiveness significantly increases for particles smaller than a limit size which is different for different materials, typical values lying between about 10 .mu.m and 40 .mu.m. It is thus recognised that small particle sizes, say less than 5 .mu.m, are advantageous for some purposes; but such particles are not easily made.
A. Chattaway et al. (Proc. Halon Options Tech. Working Conf., Albuquerque, 1995) have described a method of making potassium carbonate or bicarbonate by spray drying a solution of the salt, making particles in the size ranges 0.2 to 3 .mu.m or 0.1 to 5 .mu.m; and such powders were found to be more efficient fire suppressants than commercially available dry chemical powders. It may be possible to make a fire suppressant by forming an aerosol of droplets of borax (Na.sub.2 B.sub.4 O.sub.7.10 H.sub.2 O) aqueous solution of size less than 5 .mu.m, and drying to form a powder.